In order to perform efficient communication, a receiving end should feed back channel information to a transmitting end. Generally, downlink channel information is transmitted from a user equipment to a base station via uplink, and uplink information is transmitted from a base station to a user equipment vie downlink. Such channel information is referred to as a Channel Quality Indicator (CQI). The channel quality information may be generated by using a variety of methods.
FIG. 1 illustrates exemplary generation and transmission of a channel quality indicator.
A user equipment measures the quality of a downlink channel, and, then, the user equipment reports a value of a channel quality indicator, which is selected based upon the measured quality, to the base station through an uplink control channel. Thereafter, based upon reported channel quality indicator, the base station performs downlink scheduling (user equipment selection, resource allocation, and so on).
More specifically, in a wireless communication system, when the base station allocates wireless (or radio) resource to the user equipment, the base station uses the channel quality indicator received from the user equipment.
The user equipment receives a pilot channel from the base station so as to calculate channel quality information, such as Signal to Interference Ratio (SIR). Then, the user equipment reports the channel quality information to the base station. Accordingly, the base station uses the channel quality information received from each user equipment so as to allocate wireless (or radio) resource to each base station.
The user equipment may periodically or aperiodically report the channel quality information to the base station. Additionally, the user equipment may also report the channel quality information to the base station through a Physical Uplink Control Channel (PUCCH) or a Physical Uplink Shared Channel (PUSCH).
Each of the Physical Uplink Control Channel and the Physical Uplink Shared Channel uses a different coding method. And, the decoding performance of the Physical Uplink Shared Channel is more excellent.
FIG. 2 illustrates physical channels that are used in a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, which is an example of a mobile communication system and a general signal transmitting method using the same.
When a power of a user equipment is turned off and then turned back on, or when a user equipment newly enters (or accesses) a cell, the user equipment performs an initial cell search process, such as synchronizing itself with the base station in step S101. For this, the user equipment may receive a P-SCH (Primary Synchronization Channel) and an S-SCH (Secondary Synchronization Channel) from the base station so as to be in synchronization with the base station, and the user equipment may also acquire information, such as cell ID. Thereafter, the user equipment may receive a Physical Broadcast Channel so as to acquire broadcast information within the cell. Meanwhile, the user equipment may receive Downlink Reference Signal (DL RS), in the step of initial cell search, so as to verify the downlink channel status. The user equipment that has completed the initial cell search may receive a Physical Downlink Control Channel (PDCCH) and a Physical Downlink Shared Channel (PDSCH) based upon the Physical Downlink Control Channel (PDCCH) information, in step S102, so as to acquire more detailed system information. Meanwhile, the user equipment that has not yet completed the initial cell search may perform a Random Access Procedure, such as in steps S103 and S106 of a later process, so as to complete the access to the base station. In order to do so, the user equipment transmits a characteristic sequence through a Physical Random Access Channel (PRACH) as a preamble (S103), and then the user equipment may receive a response message respective to the random access through the PDCCH and its respective PDSCH (S104). In case of a contention based random access, excluding the case of a handover, the user equipment may perform Contention Resolution Procedures, such as transmitting an additional Physical Random Access Channel (PRACH) (S105) and receiving a Physical Downlink Control Channel (PDCCH) and a Physical Downlink Shared Channel (PDSCH) corresponding to the PDCCH.
After performing the above-described procedures, the user equipment may receive a Physical Downlink Control Channel (PDCCH)/Physical Downlink Shared Channel (PDSCH) (S107), as a general uplink/downlink signal transmission procedure, and may then perform Physical Uplink Shared Channel (PUSCH)/Physical Uplink Control Channel (PUCCH) transmission (S108). In the recent 3GPP (3rd Generation Project Partnership), the standardization procedure for the development of an LTE system is currently in progress, and the early LTE user equipments and base stations are already under development. One of the most emphasized characteristics of all types of wireless communication systems that are currently being developed, including the LIE system, is mobility. More specifically, in order to prevent any problem in communication from occurring, due to the change in channels with respect to the movement of the user equipment, a cell to which the corresponding user equipment currently belongs and its neighboring cell should be consistently monitored, and the cell that effectively receives services shall be changed.
FIG. 3 illustrates an example showing a situation wherein the user equipment is moved (or relocated) so as to enter the coverage of a neighboring cell. As shown in FIG. 1, if the user equipment was initially communicating with cell A at time T1, and, due to the movement of the user equipment, if the user equipment has moved outside of the coverage of cell A and into the coverage of cell B, at time T2, it will be difficult for the user equipment to perform reliable communication with cell A. Accordingly, reliable communication may be available if the initial communication channel is disconnected, and if the user equipment is connected to a communication channel with cell B. Depending upon an RRC (Radio Resource Control) protocol state, when an RRC connection is configured, the user equipment may be in an RRC_CONNECTED state. And, when an RRC connection is not available, the user equipment is in an RRC_IDLE state. When the user equipment is in an RRC_IDLE state, a cell reselection for a handover may be performed, and system information may be acquired. And, when the user equipment is in an RRC_CONNECTED state, the user equipment is in a handover state.
Generally, in order to perform the above-described cell reselection or handover, the channel state of a cell to which the current user equipment belongs and the channel state of its neighboring cell should be consistently measured and monitored.
In a mobile communication system, when a packet is being transmitted, the transmitted packet is transmitted through a wireless (or radio) channel. Therefore, signal distortion may occur during the transmission process. In order to allow the receiving end to correctly (or properly) receive such distorted signal, the receiving end is required to be informed of (or to figure out) the respective channel information, so as to compensate for the distortion occurring in the transmitted signal from the received signal in accordance with the informed channel information. A general method that is used in order to be informed of (or to figure out) the channel information, a signal known to both the receiving end and the transmitting end is transmitted. Then, when the transmitted signal is received through the corresponding channel, information on the corresponding channel is obtained based upon the distortion degree occurring in the received signal. Herein, the signal that is known to both the receiving end and the transmitting end is referred to as a pilot signal or a Reference Signal (hereinafter also referred to as RS). Recently, in most of the mobile communication systems, when a packet is being transmitted, in case data are transmitted and received by using multiple antennas, a reference signal exists for each transmission antenna. And, by being information of the channel status between each transmission antenna and each reception antenna, a correct signal may be received.
In the current LTE system, the channel quality state between a user equipment and a cell is measured by using a RSRP (Reference Signal Receiving Power). More specifically, the user equipment accumulates pilot signals transmitted from each cell with respect to a designated time and bandwidth, so as to measure a received signal power of each cell.
In an environment having a cell, to which the corresponding user equipment belongs, co-exist with a plurality of neighboring cells, the user equipment measures the RSRP for each of the serving cell, to which the user equipment belongs, and the neighboring cells.
Since the user equipment is provided with mobility, the channel quality for each cell may vary depending upon the respective time. And, at a specific time point, a cell having better channel quality than that of the cell, to which the current user equipment belongs, may be detected. When the user equipment detects a cell having better channel quality than that of its serving cell for a predetermined period of time, the user equipment may become communicating with the cell having the better channel quality.
The standardization process for the current 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) system is practically completed. And, presently, the standardization process for the LTE-A (Long Term Evolution Advance) for supporting a more enhanced transmission rate than that of the LTE system is under development.
In the LTE-A system, in order to enhance the transmission rate, Carrier Aggregation (CA) is being applied. Hereinafter, Carrier Aggregation will be described in detail.
FIG. 4 illustrates an example showing an uplink band and a downlink band in a general FDD type wireless mobile communication system. As shown in FIG. 3, in a general FDD type wireless mobile communication system, data transmission and reception is realized through one downlink band of 20 MHz and its corresponding uplink band.
Recently, however, in order to support larger uplink and downlink bandwidths, and in order to support a larger number of uplink and downlink bandwidths, a system that is configured of larger uplink and downlink bandwidths by collecting (or grouping) multiple Compnent Carriers (CCs) is being researched and developed.
FIG. 5 illustrates an exemplary bandwidth of the FDD type wireless mobile communication system, when applying carrier aggregation. Most particularly, FIG. 4 shows an example wherein 5 component carriers each corresponding to 20 MHz are grouped in each of the uplink and the downlink, so as to support a bandwidth of 100 MHz. In the 3GPP LTE-A (LTE-Advanced) system, which is currently being designed, multiple component carriers each having a maximum bandwidth of 20 MHz are grouped so as to be capable of supporting a larger uplink and downlink bandwidth.
As shown in FIG. 2, it may be understood that the conventional LIE system uses only one component carrier, and the RSRP for the signal that is being transmitted from the corresponding carrier may be used as a reference value for measuring channel quality.
In a system using multiple component carriers, in order to measure the channel quality, the system should decide a specific component carrier, so as to use the signal being transmitted from the decided component carrier to measure channel quality.
Firstly, when the user equipment performs channel quality measurement with respect to the serving cell, to which the corresponding user equipment belongs, the user equipment should decide which component carrier it will be using in order to measure the channel quality.
For example, as shown in FIG. 4, when it is assumed that the number of component carriers existing in the cell, to which the corresponding user equipment belongs, is equal to 5, the user equipment may be allocated with a maximum of 5 component carriers. Herein, the 5 component carriers may all be allocated to the user equipment, or only a selected number of component carriers among the 5 component carriers may be allocated to the user equipment so as to be used. In this case, the user equipment shall select the component carriers it will be using in order to perform the channel quality state measurement of the serving cell, to which the corresponding user equipment belongs.
Secondly, in order to perform effective cell reselection and handover, the user equipment is required to measure and monitor the channel quality of a neighboring cell. Accordingly, when the neighboring cell uses multiple component carriers so as to transmit a signal, a specific component carrier shall be selected among the multiple component carriers, in order to measure the channel quality state. A Neighbor Cell List (NCL), which includes information on the neighboring cells, corresponds to information that is notified to the user equipment by the serving cell, to which the corresponding user equipment belongs. Therefore, in the LTE system, the Neighbor Cell List is a selective option. Generally, in the LTE system, information on an Intra-frequency cell is not given to the user equipment. And, for an Inter-frequency)/RAT (Radio Access Technology) cell, information on the frequency, to which the corresponding cell belongs, and information on the specific system that is being used is provided to the user equipment.
When an intra-frequency neighboring cell using a frequency identical to that used by the cell, to which the user equipment belongs, uses carrier aggregation as the above-described information on the neighboring cell, the user equipment is unaware of the component carrier that is being used for transmitting a signal. Also, in case carrier aggregation is used for the inter-frequency/RAT neighboring cell, the user equipment is not informed of the information on the component carrier. Therefore, a problem may occur in measuring the channel quality between the user equipment and the neighboring cell.